In the refrigeration and air-conditioning industry, there mainly are two types of evaporators used in an high-efficiency water chilling (heat pump) unit, namely a flooded evaporator and a falling film evaporator, wherein the falling film evaporator has the advantages of high heat exchange efficiency, less refrigerant filling quantity and the like in comparison with the flooded evaporator, therefore, application prospect of the falling film evaporator is more widely. The falling film evaporator includes full falling film mode and mixed falling film mode, the full falling film mode means that all of heat exchange tubes of the evaporator are in the state of falling film heat exchange, that is to say, none of heat exchange tubes are immersed in liquid refrigerant, it has the advantage of being capable to furthest reduce refrigerant filling quantity; the mixed falling film mode means that one part of heat exchange tubes of the evaporator are in the state of falling film heat exchange, the other part of heat exchange tubes are in the state of flooded heat exchange, which are immersed in liquid refrigerant, it has the advantage of fully utilizing heat exchange area since the heat exchange tubes at the bottom of the evaporator are immersed in liquid refrigerant. At present, the falling film evaporator has the following disadvantages: 1) refrigerant dripping to the heat exchange tubes at the bottom of the full falling film evaporator is insufficient, which, in turn, results in that the utilization of heat exchange area is not maximized; 2) the mixed falling film evaporator maintains a high refrigerant liquid level at the bottom thereof, and refrigerant at the bottom of the evaporator has a high oil content, these factors cause that heat transfer coefficient of the lower heat exchange tube group is low, and refrigerant filling quantity increases since a large amount of liquid refrigerant exists in the bottom of the evaporator; 3) because of design limitation, velocity field of gas refrigerant inside the evaporator is not reasonable, a region with high flow velocity concentrates on the vicinity of the refrigerant outlet, phenomenon of carrying liquid during air suction often arises, and high speed airflow easily forms turbulence for liquid refrigerant, and consequently reduces heat exchange efficiency.